Processor apparatus and method for a process measurement signal

ABSTRACT

A method and apparatus for processing a time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a valid output result corresponding to a process variable in a vessel. The method includes the steps of establishing an initial boundary signal before the process variable is locates in the vessel, storing the initial boundary signal, detecting the TDR signal, and determining a baseline signal by subtracting the initial boundary signal from the TDR signal. The method also includes the steps of establishing a signal pattern having a time range based on the width of reflection pulses in the baseline signal, comparing the baseline signal to the signal pattern until a reflection pulse in the baseline signal matches the signal pattern, determining a maximum value of the reflection pulse that matches the signal pattern, and calculating an output result based on the maximum value.

This application is a continuation-in-part of application Ser. No.08/576,554 filed Dec. 21, 1995.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a processor apparatus and method for aprocess measurement signal. More particularly, the present inventionrelates to an improved processor for time-of-flight signals to providean accurate indication of the location of an interface between a firstmedium and a second medium in a vessel.

The process and storage industries have long used various types ofequipment to measure process parameters such as level, flow,temperature, etc. A number of different techniques (such as mechanical,capacitance, ultrasonic, hydrostatic, etc.) provide measurementsolutions for many applications. However, many other applications remainfor which no available technology can provide a solution, or whichcannot provide such a solution at a reasonable cost. For manyapplications that could benefit from a level measurement system,currently available level measurement systems are too expensive.

In certain applications, such as high volume petroleum storage, thevalue of the measured materials is high enough to justify high costlevel measurement systems which are required for the extreme accuracyneeded. Such expensive measurement systems can include a servo tankgauging system or a frequency modulated continuous wave radar system.

Further, there are many applications that exist where the need tomeasure, level of the product is high in order to maintain productquality, conserve resources, improve safety, etc. However, lower costmeasurement systems are needed in order to allow a plant to instrumentits measurements fully.

There are certain process measurement applications that demand otherthan conventional measurement approaches. For example, applicationsdemanding high temperature and high pressure capabilities during levelmeasurements must typically rely on capacitance measurement. However,conventional capacitance measurement systems are vulnerable to errorsinduced by changing material characteristics. Further, the inherentnature of capacitance measurement techniques prevents the use of suchcapacitance level measurement techniques in vessels containing more thanone fluid layer.

Ultrasonic time-of-flight technology has reduced concerns regardinglevel indications changing as material characteristics change. However,ultrasonic level measurement sensors cannot work under hightemperatures, high pressures, or in vacuums. In addition, suchultrasonic sensors have a low tolerance for acoustic noise.

One technological approach to solving these problems is the use ofguided wave pulses. These pulses are transmitted down a dual probetransmission line into the stored material, and are reflected from probeimpedance changes which correlate with the fluid level. Processelectronics then convert the time-of-flight signals into a meaningfulfluid level reading. Conventional guided wave pulse techniques are veryexpensive due to the nature of equipment needed to produce high-quality,short pulses and to measure the time-of-flight for such short timeevents. Further, such probes are not a simple construction and areexpensive to produce compared to simple capacitance level probes.

Recent developments by the National Laboratory System now make itpossible to generate fast, low power pulses, and to time their returnwith very inexpensive circuits. See, for example, U.S. Pat. Nos.5,345,471 and 5,361,070. However, this new technology alone will notpermit proliferation of level measurement technology into process andstorage measurement applications. The pulses generated ly this newtechnology are broadband, and also are not square wave pulses. Inaddition, the generated pulses have a very low power level. Such pulsesare at a frequency of 100 MHz or higher, and have an average power levelof about 1 nW or lower. These factors present new problems that must beovercome to transmit the pulses down a probe and back and to process andinterpret the returned pulses.

First, a sensor apparatus must be provided for transmitting these lowpower, high frequency pulses down a probe and effecting their return.Such appropriate sensor apparatus is described in a copending U.S.patent application Ser. No. 08,574,818, entitled SENSOR APPARATUS FORPROCESS MEASUREMENT, filed Dec. 19, 1995 and a copending U.S. patentapplication Ser. No. 08/735,736 entitled SENSOR APPARATUS FOR PROCESSMEASUREMENT, filed Oct. 23, 1996, the disclosures of which are herebyexpressly incorporated by reference into the present application.

The sensor apparatus is particularly adapted for the measurement ofmaterial levels in process vessels and storage vessels, but is notlimited thereto. It is understood that the sensor apparatus may be usedfor measurement of other process variables such as flow, composition,dielectric constant, moisture content, etc. In the specification andclaims, the term "vessel" refers to pipes, chutes, bins, tanks,reservoirs or any other storage vessels. Such storage vessels may alsoinclude fuel tanks, and a host of automotive or vehicular fluid storagesystems or reservoirs for engine oil, hydraulic fluids, brake fluids,wiper fluids, coolant, power steering fluid, transmission fluid, andfuel.

The present invention propagates electromagnetic energy down aninexpensive, signal conductor transmission line as an alternative toconventional coax cable or dual transmission lines. The Goubau linelends itself to applications for a level measurement sensor where aneconomical rod or cable probe (i.e., a one conductor instead of a twinor dual conductor approach) is desired. The single conductor approachenables not only taking advantage of new pulse generation and detectiontechnologies, but also constructing probes in a manner similar toeconomical capacitance level probes.

The present invention specifically relates to a signal processorapparatus for processing and interpreting the returned pulses from theconductor. Due to the low power, broadband pulses used in accordancewith the present invention, such signal processing to provide ameaningful indication of the process variable is difficult.

Conventional signal processing techniques use only simple peak detectionto monitor reflections of the pulses. The present invention providessignal processing circuitry configured for measurement of thetime-of-flight of very fast, guided wave pulses. Techniques used insimilar processes, such as ultrasonic level measurement are vastlydifferent from and are insufficient for detection of guidedelectromagnetic wave pulses due to the differences in signalcharacteristics. For example, ultrasonic signals are much noisier andhave large dynamic ranges of about 120 dB and higher Guidedelectromagnetic waves in this context are low in noise and have lowdynamic ranges (less than 10:1) compared to the ultrasonic signals, andare modified by environmental impedances. The signal processor of thepresent invention is configured to determine an appropriate reflectionpulse of these low power signals from surrounding environmentalinfluences.

Standard electromagnetic reflection measurements are known as timedomain reflectometry (TDR). TDR devices for level measurement requirethe measuring of the time of flight of a transit pulse and asubsequently produced reflective pulse received at the launching site ofthe transit pulse. This measurement is typically accomplished bydetermining the time interval between the maximum amplitude of thereceived pulse. The determination of this time interval is done bycounting the interval between the transmitted pulse and the receivedpulse.

The present invention provides an improved signal processor fordetermining a valid reflective pulse signal caused by an interface ofmaterial in contact with a probe element of a sensor apparatus. Theprocessor apparatus of the present invention is particularly useful forprocessing high speed, low power pulses as discussed above. In thepreferred embodiment of the signal processor apparatus, processing isperformed based on a digital sampling of an analog output of thereflective pulses. It is understood, however, that similar signalprocessing techniques can be used on the analog: signal in real time.

It is well known that variations in operating conditions such asenvironmental variations like temperature, humidity, and pressure; powervariations like voltage, current, and power; electromagnetic influenceslike radio frequency/microwave radiated power which creates biases onintegrated circuit outputs; and other conditions such as mechanicalvibration can induce undesired drifts of electronics parameters andoutput signals. The present invention provides a processing means andmethod for compensating for signal drifts caused by these operatingconditions.

According to one aspect of the present invention, a method is provided,for processing a time domain reflectometry (TDR) signal to generate avalid output result corresponding to a process variable in a vessel. Themethod includes the steps of establishing an initial boundary signalbefore the process variable is located in the vessel; storing theinitial boundary signal and detecting a TDR signal. The method alsoincludes the steps of determining a baseline signal by subtracting theinitial boundary signal from the TDR signal, establishing a signalpattern having a time range based on the width of reflection pulses inthe baseline signal and comparing the baseline signal to the signalpattern until a reflection pulse in the baseline signal matches thesignal pattern.

According to another aspect of the present invention, a method forprocessing a time domain reflectometry (TDR) signal to generate a validoutput result corresponding to a process variable in a vessel includesthe steps of establishing an initial boundary signal before the processvariable is located in the vessel, storing the initial boundary signal,and detecting the TDR signal. The method further includes the steps ofdetermining a point on the initial boundary signal and a correspondingpoint on the TDR signal, calculating a correction factor by subtractingthe point on the initial boundary signal from the corresponding point onthe TDR signal, and adding the correction factor to the TDR signal toestablish a valid TDR signal. The method further includes the step ofdetermining a baseline signal by subtracting the initial boundary signalfrom the valid TDR signal.

In the illustrated embodiment, the first processing method includes thesteps of establishing a threshold voltage prior to comparing thebaseline signal to the signal pattern and converting negative-goingcomponents of the reflection pulses to positive-going components.Further, in the first processing method the step of establishing asignal pattern includes the step of determining at least four pointswithin the time range in proximity to the threshold voltage and the stepof comparing the baseline signal to the signal pattern includes the stepof searching for a reflection pulse where the four points are on thereflection pulse in proximity to the threshold voltage.

In another illustrated embodiment, the second processing method includesthe step of converting the correction factor to a positive value priorto adding the correction factor to the TDR signal to establish the validTDR signal.

In both illustrated methods, the processing methods include the steps ofdetermining a maximum value of the baseline signal and calculating anoutput result based on the maximum value.

According to a further aspect of the present invention, an apparatus isprovided for processing a time domain reflectometry (TDR) signal havinga plurality of reflection pulses to generate a valid output resultcorresponding to a process variable in a vessel. The apparatus includesmeans for establishing an initial boundary signal before the processvariable is located in the vessel, means for storing the initialboundary signal, means for detecting the TDR signal, and means fordetermining a baseline signal by subtracting the initial boundary signalfrom the TDR signal. The apparatus further includes means forestablishing a signal pattern having a time range based on the width ofreflection pulses in the baseline signal and means for comparing thebaseline signal to the signal pattern until a reflection pulse in thebaseline signal matches the signal pattern.

According to yet another aspect of the present invention an apparatus isprovided for processing a time domain reflectometry (TDR) signal havinga plurality of reflection pulses to generate a valid output resultscorresponding to a process variable in a vessel. The apparatus includesmeans for establishing an initial boundary signal before the processvariable is located in the vessel, means for storing the initialboundary signal, and means for detecting the TDR signal. The apparatusfurther includes means for determining a point on the initial boundarysignal and a corresponding point on the TDR signal, means forcalculating a correction factor by subtracting the point on the initialboundary signal from the corresponding point on the TDR signal, meansfor adding the correction factor to the TDR signal to establish a validTDR signal, and means for determining a baseline signal by subtractingthe initial boundary signal from the valid TDR signal.

The illustrated embodiments of the apparatus also include means fordetermining a maximum value of the baseline signal and means forcalculating an output result based on the maximum value.

Additional objects, features, and advantages of the invention willbecome apparent to those skilled in the art upon consideration of thefollowing detailed description of the preferred embodiment exemplifyingthe best mode of carrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figuresin which:

FIG. 1 is a diagrammatical view illustrating a single conductor materiallevel sensor for measuring a level of a process variable such as aliquid in a vessel, and illustrating a block diagram of the pulsetransmitter and receiver and the processing circuitry for determiningthe level of the process variable;

FIG. 2 is an analog signal output of the time domain reflectometry (TDR)signal generated by the transmitter and a receiver;

FIG. 3 is an analog output signal indicating an initial boundarycondition of the inside of the vessel before the process variable islocated in the vessel;

FIG. 4 is a time aligned analog TDR output signal:

FIG. 5 is an analog derivative signal of the time aligned TDR signal ofFIG. 4;

FIG. 6 is an analog baseline signal generated when the initial boundarysignal of FIG. 3 is subtracted from the time aligned TDR output signalof FIG. 4;

FIG. 7 is an analog signal of a derivative of the baseline signal ofFIG. 6;

FIG. 8 is a flow chart illustrating the steps performed by the processorapparatus of the present invention to determine an actual, valid levelindication of the process variable based on a reflective pulse caused bythe process variable;

FIG. 9 is an analog baseline signal corresponding to the signal shown inFIG. 6 illustrating the pattern recognition technique of determining thevalid baseline signal;

FIG. 10 is an analog initial boundary or probe map time aligned signalcorresponding to FIG. 3;

FIG. 11 is an analog illustration of the drift of a real time initialboundary signal relative to the initial boundary signal shown in FIG. 10caused by variations in operating conditions;

FIG. 12 is an analog illustration of a baseline signal after theapplication of a correction factor according to the present invention tocompensate for the drift in the signal shown in FIG. 11;

FIG. 13 is a segment of the flow chart illustrated in FIG. 8incorporating the steps performed by the processor apparatus of thepresent invention to determine and apply the correction factor and touse the pattern recognition technique to determine an actual, validlevel indication of the process variable based on a reflective pulsecaused by the process variable; and

FIG. 14 is a flow chart expanding the steps performed in block 250 inFIG. 13 for calculating and adding the correction factor to the initialboundary signal.

DETAILED DESCRIPTION OF DRAWINGS

Referring now to the drawings, FIG. 1 provides a diagrammaticalillustration of operation of the surface wave transmission line sensorapparatus for process measurement. The apparatus 10 is adapted for usewith level measurement of a process variable such as an interfacebetween a first medium 11 and a second medium 12. located within astorage vessel 14. Illustratively, the first medium 11 is air and thesecond medium 12 is a process variable such as a liquid or othermaterial.

The present invention includes a mechanical mounting apparatus 16 forsecuring a single conductor transmission line or probe element 18 to asurface 20 of the vessel 14. The mechanical mounting apparatus 16enables a transceiver 22 to transmit pulses onto the probe element 18 inthe direction of arrow 24. Once the pulses reach an interface 26 betweenthe first medium 11 and the second medium 12, such as a top surface ofliquid, a reflective pulse is returned back up the probe element 18 inthe direction of arrow 28.

The transceiver 22 is coupled to processing circuitry which detects thereflected pulses to interpret the return pulses and to generate anoutput signal indicating the level of second medium 12 in the vessel 14.Preferably, the transceiver 22 transmits broadband pulses at very lowaverage power levels such as about 1 nW or less, or 1 μW or less peakpower. The frequency of the pulses is preferably about 100 MHz orgreater.

The transceiver 22 includes a transmit pulse generator 30 which generates a series of the high frequency pulses and transmits these pulses via acable 37 to mounting 16. Transceiver 22 also includes a sequential delaygenerator 32 coupled to the transmit pulse generator 30. A sample pulsegenerator 34 is coupled to the sequential delay generator 32. A sampleand hold buffer 36 is coupled to sample pulse generator 34 and to thecable 37. Illustratively, transceiver 22 is a micropower wide bandimpulse radar transmitter developed by the Lawrence Livermore NationalLaboratory located at the University of California located in Livermore,Calif. It is understood, however, that other transceivers 22 may also beused with the signal processor apparatus of the present invention.

As discussed above, the mounting apparatus 16 must be specially designedto transmit and receive the low power, high frequency pulses. Theabove-referenced copending applications, the disclosures of which areexpressly incorporated by reference, provide a suitable mountingapparatus 16 for transceiver 22. It is understood that the electronicsand processing circuitry may be located at a remote mounting locationspaced apart from the mounting apparatus 16.

An output from transceiver 22 on line 38 is coupled to an amplifier 40.An output from amplifier 40 provides a TDR analog signal on line 42.Although the preferred embodiment of the present invention uses adigital sampling system and processes digital signals related to theanalog output signals, it is understood that a processor apparatus inaccordance with the present invention may be built to process the analogsignal directly.

In the present invention, an analog-to-digital converter 44 is coupledto amplifier 40. An output of the analog-to-digital converter 44 iscoupled to an input of microprocessor 46. In the illustrated embodiment,microprocessor 46 is a MC68HC711E9 microprocessor available fromMotorola. It is understood, however, that any other suitablemicroprocessor may be used in accordance with present invention.Microprocessor 46 is used to implement both a fast clock and a slowclock. A PRF clock implemented by microprocessor 46, which is a squarewave at about 2 MHz, is coupled to transmit pulse generator 30. Themicroprocessor 46 also implements a sync oscillator, which isillustratively a square wave having a frequency of about 40 Hz. The syncoscillator is coupled to sequential delay generator 32.

Microprocessor 46 is also coupled to RAM 48 and to EEPROM 50. An outputterminal of microprocessor 46 is coupled to an output 52.Illustratively, output 52 provides a 4-20 nA output signal to provide anindication of the level of the interface 26 between the first medium 11and the second medium 12.

The TDR analog signal from amplifier 40 is an equivalent time signal(ETS) of the real time signal traveling on the transmission line system.The ETS is expanded in time by way of digital sampling, thereby enablingthe use of conventional hardware for signal conditioning and processing.The signal processor of the present invention provides means fordetermining a valid pulse reflection, whether in real time or from theETS. These results allow flexibility to determine information relatingto the position of mediums 11 and 12 relative to a top surface 20, abottom surface 21, a sensor launch plate, or an end 19 of the probeelement 18. The process material positional information is derived fromsignal reflections caused by impedance discontinuities on thetransmission line and subsequent signal processing.

The signal responses of a transmission line which includes cable 37,mounting 16, and probe element 18 are dependent upon the inherenttransmission design characteristics and impedance changes created bychanging boundary conditions. These boundary conditions are used todetermine changes in the sensor environment and are directly orindirectly related to the amount or position of the bulk processmaterials being measured. The impedance of the sensor at a givenlocation can change with variations of the sensor's environment orboundary condition due to interaction of the sensor, its signal, and itssurroundings.

An example of a time domain reflectometry (TDR) analog signal fromamplifier 40 is illustrated in FIG. 2. In FIG. 2, the first largevoltage fluctuation or pulse 54 is generated by the impedance change inthe mounting 16. In the preferred embodiment, the mounting 16 providesthis impedance change as a reference reflective pulse. The secondreflective pulse 56 in FIG. 2 is generated by an inherent interferencewithin vessel 14. This interference reflection 56 may be caused by aladder, door, weld seam, material buildup, or other internal factor fromvessel 14. The third reflective pulse 58 is provided by the interface 26between the first medium 11 and the second medium 12. The fourthreflective pulse 60 is generated by an end 19 of probe element 18.

The present invention initializes the signal processing function bycharacterizing or recording sensor performance at a given time or underknown boundary conditions so that this initial characterization can beused as an initial boundary condition. In other words, a reference orinitial boundary signal is measured and stored before the first andsecond mediums 11 and 12 are placed in the vessel 14.

An example of an initial boundary signal (I.B.) is illustrated in FIG.3. The initial boundary signal is used to help determine a validimpedance change induced reflective pulse caused by interface 26 betweenfirst medium 11 and second medium 12. In FIG. 3, the initial voltagepeak or reflective pulse 62 is caused by the interference in the vessel14. Pulse 62 of FIG. 3 corresponds to pulse 56 in FIG. 2. Pulse 64 inFIG. 3 corresponds to the end 19 of probe element 18.

The sensor characterization may include factory calibration,environmental characterization or probe mapping, and sensorrecharacterization, or recalibration. The characterization can be donein such a way to permit use of only one or a combination ofinitialization procedures to provide optimum performance. Thecharacterization of the sensor and its signals inside or outside of itsinstallation environment such as the mounting in the vessel 14 arereferred to as its initial boundary conditions.

Factory calibration may include characterizing sensor performance in astable, known environment which provides a baseline for the systemperformance while neglecting the influences and effects that areencountered in field installation. A field installation, such asmounting the sensor in a tank or vessel 14, can present an environmentfor new boundary conditions to the sensor caused by the vessel orpermanent contents of the vessel which influence the sensor response dueto interaction of the sensor with these vessel contents.

The present invention provides either an automatic recharacterization ora manual recharacterization of the sensor which can be performed tore-establish a new baseline or probe map which enables theseenvironmental changes to be accounted for in determining the validsignal indicating the desired process variable.

A second phase of the signal processor of the present invention involvesdetecting the pulse reflection produced by a valid signal response ofthe impedance change along a conductor. In other words, the processorapparatus locates the impedance pulse reflection caused by the interface26 between the first medium 11 and the second medium 12 in contact withthe probe element 18. A number of mathematical techniques can be used todetermine the positional information due to impedance changes whichgenerate a signal reflection related in time to the position of thecause of the impedance change along the probe element 18.

Detection of impedance changes may include one or more of the followingtechniques applied to the TDR analog output signal illustrated in FIG.2. One detection method is a peak amplitude detection of a Time AlignedTDR signal which is illustrated in FIG. 4. In other words, the signal ofFIG. 4 is shifted so that time zero is set as the time of the initialreflecting pulse 54 provided by the impedance change at the mounting 16.In FIG. 4, the first reflection pulse 66 is caused by the interferencewithin vessel 14. Second reflection pulse 68 is caused by interface 26.The third reflection pulse 70 is caused by end 19 of the probe element18.

Another detection technique is to determine the first zero crossingafter the positive peak of a first derivative signal of the Time AlignedTDR signal of FIG. 4. This derivative signal is illustrated in FIG. 5.Again, the first reflection pulse 72 is caused by the interferencewithin vessel 14. The second refleciton pulse 74 is caused by interface26, and the third refleciton pulse 76 is caused by end 19 of probeelement 18. Using this technique, the processor apparatus determines themaximum absolute value of the peak reflective pulse, which isillustratively at location 78. If the absolute maximum was a negativevalue, the preceding zero crossing at location 80 is determined to bethe location of interface 26. If the absolute maximum was a positivepeak, the next subsequent zero crossing is used as the indication ofinterface 26.

Yet another technique for determining the valid interface 26 is the useof a baseline signal. The baseline signal is illustrated in FIG. 6. Thebaseline signal is determined by subtracting the initial boundary signalof FIG. 3 from the Time Aligned TDR signal of FIG. 4. Therefore, thepulse reflection 66 caused by the interference within vessel 14 iscancelled by the initial boundary pulse reflection 62. In FIG. 6, theinitial pulse reflection 82 is therefore caused by the interface 26between the first medium 11 and the second medium 12. Reflective pulse84 is caused by the end 19 of probe element 18. The processor determinesthe time of the greatest positive peak 86 as the pulse refleciton causedby interface 26.

Still another technique for determining the actual position of interface26 is to use the first derivative signal of the baseline signal of FIG.6. The derivative of the baseline signal is illustrated in FIG. 7.Again, the first reflection pulse 88 is caused by the interface 26between first medium 11 and second medium 12. The second reflecitonpulse 90 is caused by end 19 of probe element 18. The processordetermines the peak absolute value 92 of the pulse refleciton 88. Sincethe peak absolute value is associated with a negative voltage, theprocessor proceeds to the first proceeding zero crossing 94 as the timefor the interface 26. If the maximum absolute value was a positive peak,the next subsequent zero crossing is used as the interface level.

Some embodiments of the present invention use a combination of two ormore of the above-cited techniques to verify the data related to thevalid detection of interface 26. The short term history of the signalcan also be used to substantiate the validity of any change in positionof the interface 26 and to verify that this change is possible withinthe process condition presently being used in the vicinity of thesensor.

In a preferred embodiment of the present invention, the processordetermines the location of the valid impedance discontinuity caused byinterface 26 between first medium 11 and second medium 12 using each ofthe four techniques or methods discussed above. Each method is assigneda weighted factor. In the illustrated embodiment, the baseline signalcalculation illustrated in FIG. 6 is assigned a weighted factor of 1.1,while the other three techniques are assigned a weighted factor of 1.0.These weighted factors provide means for showing the degree of agreementamong the four methods. If the calculated boundary conditions asdetected by the sensor creates a conflict among the four detectionmethods such that there is not a substantial agreement of all fourmethods, then a valid result is dependent upon whether there issubstantial agreement between two or three of the detection methods. Ifthere is substantial deviation in the detection of the valid impedancepulse by all four methods, then the method having the highest weightedfactor is used as the valid detection.

In the present invention, the microprocessor 46 is programmed withsoftware to calculate the position of the valid impedance change causedby interface 26 using each of the four methods discussed above. FIG. 8illustrates the steps performed by the microprocessor 46 of the presentinvention to determine the valid signal. The microprocessor 46 is firstinitialized as illustrated at block 100. Operation mode of the signalprocessor is illustrated at block 102.

The first operational mode is to set and store the initial boundary(I.B.) signal illustrated in FIG. 3. This initial boundary signal isgenerated before the process material is placed in vessel 14.Microprocessor 46 first receives an input initial boundary signal asillustrated at block 104. The data is then time aligned based on theinitial impedance change caused by the mounting 16 as illustrated asblock 106. Microprocessor 46 then stores the time aligned data relatedto the initial boundary conditions in the EEPROM 50 as illustrated atblock 108. Once the initial boundary signal is stored, microprocessor 46returns to operation mode at block 102.

In one embodiment, the signal processor of the present invention mayestablish the initial boundary conditions manually only during initialinstallation of the sensor apparatus 10 into the vessel 14. In anotherinstance, the initial boundary conditions may be updated atpredetermined times during operation of the signal processor.

During normal operation of the signal processor, microprocessor 46receives an input TDR signal as illustrated at block 110. This input TDRsignal is a digital representation from analog-to-digital converter 44of the TDR analog signal illustrated in FIG. 2. Although reference willbe made to the analog signals in FIGS. 2-7, it is understood that themicroprocessor 46 of the present invention uses the digitalrepresentation of these signals. It is also understood that an analogprocessor may be used to process the analog signals in accordance withthe present invention.

Microprocessor 46 next provides a time alignment of the TDR signal asillustrated at block 112. In other words, microprocessor 46 time shiftsthe input TDR signal so that the time zero begins at the location of theinterface of mounting 16 which is indicated by the initial largereflection pulse 54 shown in FIG. 2.

In the illustrated embodiment, microprocessor 46 uses four differentdetection methods to locate a valid pulse reflection indicative of theinterface 26 between the first medium 11 and the second medium 12. In afirst method, microprocessor 46 detects a peak reflection pulse of thetime aligned TDR signal (illustrated in FIG. 4) as illustrated in block114 of FIG. 8. Peak 71 in FIG. 4 is the valid reflection pulsecorresponding to interface 26. However, the peak detection step in thisexample would determine that peak 115 is the valid peak. Peak 115actually corresponds to interference in vessel 14 to be the valid pulse.This explains why the peak detection method of the time aligned TDRsignal, when used alone, may produce some inaccuracies. Microprocessor46 then determines a time corresponding to the position of the maximumpulse value as illustrated at block 116 in FIG. 8. The time value isthen converted to a distance between the top surface 20 of vessel 14 andthe interface 26. This step is illustrated at block 118. This distanceresult calculated using the first detection method is then stored.

It is understood that once a time position of an impedance change on asensor has been derived, there are a number of techniques that can beused to convert the detected time to a distance equivalent position ofthe interface 26 of the process variable. The time intervals between theimpedance changes have a mathematical relationship such that the timerelation between the impedance change is proportional to the speed oflight and a continuous function of the relative dielectric constants ofthe subject materials. If the first medium 11 is air, the dielectricconstant is substantially equal to 1.0. The subject time of the intervalcan then be corrected by applying the continuous functional relationrelative to the material dielectric and the environmental surroundings.

Other techniques such as using a sensor or conductor of a known lengthand then using the relationship changes of the pulse travel times form asubject material interface to an end 19 of the probe element 18 may beused. In other words, once the location of the valid impedance pulse isdetermined, a time or distance between the impedance interface and theend 19 of probe element 18 can be used to determine the level of theinterface 26. In the case of a sensor having a known length,differential time intervals from a material interface 26 to end 19 ofthe probe element 18 changes proportionally with the thickness of thesubject material 12 divided by a continuous functional relationship ofthe material dielectric constant. Provided the probe element 18 has afixed location relative to the vessel 14, the material level orthickness of the material is an offset relative to sensor position. Thisposititional relationship is determined using a simple mathematicalequations.

Similarly, the velocity of a pulse traveling on a sensor passing throughmultiple material layers can be used to determine the level of eachmaterial, provided the relative dielectric constant of each material isknown. When the sensor has a fixed location relative to vessel 14, theposition of each material can be determined as a function of the timedifferential, with an offset to the sensor position. A sensor can alsobe designed having markers at known distances to create signalreflections that can be used for calibration and/or determining materialdielectric values.

Microprocessor 46 also calculates a derivative of the time aligned TDRsignal as illustrated at block 120. An analog representation of thisderivative signal is illustrated in FIG. 5. Microprocessor 46 thendetermines the location of a first zero crossing adjacent an absolutemaximum value of the signal. If the maximum is obtained from a positivevalue, microprocessor 46 determines the next subsequent zero crossingafter the positive peak. If the absolute maximum was obtained from anegative value, the microprocessor 46 determines the first zero crossingprior to the detected absolute maximum. This step is illustrated atblock 122. Microprocessor 46 then determines a time value correspondingto the detected zero crossing as illustrated at block 124. This timevalue is then converted to a distance corresponding to the level of theinterface 26 between first medium 11 and second medium 12 as illustratedat block 126. The distance calculated using the second detection Methodis then stored.

In the third detection method, the microprocessor 46 calculates abaseline (BL) signal by subtracting the initial boundary signal storedin EEPROM 50 (FIG. 3) from the time aligned TDR signal which isillustrated in analog form in FIG. 4 as illustrated at block 128. Thisbaseline signal is illustrated in analog form in FIG. 6. Microprocessor46 then determines a location of the positive maximum value of thebaseline signal as illustrated at block 130. This positive maximum valueis illustrated at location 86 in FIG. 6. Microprocessor 46 nextdetermines the time value corresponding to the detected positive maximumvalue as illustrated at block 132. Microprocessor 46 then converts thetime value to a distance change indicating the location of interface 26between the first medium 11 and second medium 12 as illustrated at block134. The distance calculated using the third detection methods is thenstored.

In the fourth detection method, Microprocessor 46 generates a firstderivative of the baseline signal as illustrated at block 136. An analogrepresentation of the first derivative of the baseline signal isillustrated in FIG. 7. Microprocessor 46 then determines a location of azero crossing adjacent an absolute maximum value as illustrated at block138. If the absolute maximum comes from a positive value, the nextsubsequent zero crossing is used. If the absolute maximum is from anegative value, the first preceding zero crossing is used as a locationof interface 26. Microprocessor 46 then determines the time position ofthe zero crossing at block 140. In the FIG. 7 example, the firstpreceding zero crossing 94 adjacent negative peak 92 is used as the timeposition. Microprocessor 46 then determines the time change asillustrated at block 142. This time change is then converted to adistance change as illustrated at block 144 to provide an indication ofthe level of the interface 26 between the first medium 11 and secondmedium 12. This distance change calculated using the fourth detectionmethod is then stored.

Microprocessor 46 next checks the validity of the detected distancesfrom each of the four methods discussed above as illustrated at block146. Each of the distance changes is rounded to a predeterminedsensitivity level, for example, one millimeter. If all four storedresults from each of the four methods are the same, microprocessor 46determines that a valid output has been determined. Therefore,microprocessor formats the output into an appropriate form and sends theresult to the output 52 as illustrated at block 150.

If the four stored results from the four detection methods aredifferent, microprocessor 46 then takes into account weighted factorsestablished for each of the detection methods as illustrated at block152. At this point, microprocessor 46 may compare the four stored methodresults to a previous result. If any of the four stored results deviatesfrom the previous result by more than a predetermined amount, themicroprocessor 46 may disregard such a stored result. Microprocessor 46provides a summation of the weighted results as illustrated at block154. Examples of this summation by microprocessor 46 are provided below.Microprocessor 46 then selects the most appropriate distance as thevalid impedance reflection from interface 26 using the weighted resultsat block 156. Microprocessor 46 then outputs this selected result atblock 150.

Three different examples are provided to illustrated the effect of theweighted factors on the process measurement.

EXAMPLE 1

    ______________________________________                                   Selected    Method    X (cm)        W.F.   Result    ______________________________________    Peak TDR  29.0          1.0    Der. TDR  36.9          1.0    Max. BL   37.1          1.1    37.1    Der. BL   37.3          1.0    ______________________________________

EXAMPLE 2

    ______________________________________                                   Selected    Method    X (cm)        W.F.   Result    ______________________________________    Peak TDR  36.9          1.0    Der. TDR  37.3          1.0    37.3    Max. BL   37.1          1.1    Der. BL   37.3          1.0    ______________________________________

EXAMPLE 3

    ______________________________________                                   Selected    Method    X (cm)        W.F.   Result    ______________________________________    Peak TDR  37.1          1.0    Der. TDR  37.3          1.0    Max. BL   37.1          1.1    37.1    Der. BL   37.3          1.0    ______________________________________

In Example 1, each of the detected results for the level or distance Xof the interface 26 is different. In this instance, the greatestweighted factor indicates that the maximum detected baseline value isused. Therefore, the selected result by microprocessor 46 is 37.1 cm.

In Example 2, the maximum baseline method still indicates a distance of37.1 cm. However, both the derivative of the TDR signal method and thederivative of the baseline signal method provided a result of 37.3 cm.Therefore, the distance of 37.3 cm has a weighted factor of 2.0 when thetwo identical results are added together. Distance 36.9 cm from the peakTDR signal method has a weighted factor of 1.0. Distance 37.1 due to themaximum baseline method has a weighted factor of 1.1. Therefore,microprocessor 46 selects the greatest weighted factor of 2.0 and thecorresponding distance result of 37.3 cm during the selection step atblock 156 in FIG. 8.

In Example 3, both the peak TDR method and the maximum baseline methodprovided a distance result of 37.1 cm. The derivative TDR method and thederivative baseline method both produced a result of 37.3 cm. Therefore,the distance 37.1 has a weighted factor of 2.1, while the distance 37.3cm has a weighted factor of 2.0. Therefore, microprocessor 46 selectsthe result of 37.1 cm during the selection step at block 156.

It is understood that other detection techniques may be used inaccordance with the present invention. In addition, one of the otherdetection techniques may be applied the highest weighted factor, ifdesired. In an alternate embodiment, each of the detection techniquesmay be assigned a different weighted factor. Such weighted factors areselected and applied on the basis of application knowledge andexperience.

A further technique for determining the valid interface 26 is patternrecognition using the baseline signal illustrated in FIG. 6. The patternrecognition technique uses the entire pattern of the reflected pulse 82shown in FIG. 6 and a number of sampled points taken after a reflectedpulse 82 has reached a threshold voltage. The timing of the points mustfall within specific boundaries for the pattern to be considered valid.This technique is an improvement over existing peak detection methods inthat it protects against false readings due to signal-pulse spikesproduced by noise and other phenomena.

Referring to FIG. 9 a reflected signal 200 includes a positive-goingcomponent 202 and a negative-going component 204 (shown in broken lines)and is nearly sinusoidal in shape. The baseline reflected signal 200 iscentered about zero volts as can be seen in FIG. 6.

In the baseline method for determining the valid interface 26, thecenter of positive-going component 202 of the reflected signal 200(i.e., the process material level) is determined by identifying twopoints 206 and 208 on the positive-going component 202 of the reflectedsignal 200 with respect to a threshold voltage 210. The midpoint betweenthese points 206 and 208 is the center of the positive going component202 of the reflected signal 200. Points on the negative going component204 are replaced with zeroes.

In the pattern recognition technique the points on the negative goingcomponent 206 are not replaced with zeroes. Instead the negative pointsare converted to their absolute value using the 2's complementtechnique. The 2's complement technique is well known to those skilledin the art for determining absolute value of negative signed numbers andis described and explained in standard textbooks. See for example thetextbook Digital Concepts & Applications, published 1990 by Saunder'sCollege Publishing (a division of Holt, Rinehart and Winston) p. 225.The result of the use of the 2's complement technique is a secondpositive-going component 212 creating dual positive-going peaks 202 and212.

According to the pattern recognition technique the valid interface 26for the process material is determined by using a four (4) point patternand the dual positive-going peaks 202 and 212 of the entire reflectedpulse 200. Once the first point 206 is detected relative to thethreshold voltage 210 the second point 208, third point 214 and thefourth point 216 on the positive going peaks 202 and 212 must occurwithin specific time frames from the first point 206. The time framesare determined by the overall 218 width of the valid reflected pulse200. If the four (4) points 206, 208, 214 and 216 do not occur withinthe specific time frames then the reflected pulse 200 is consideredinvalid.

If the reflected pulse 200 is found to be valid, then the center of thefirst positive-going peak 202 (i.e. the valid interface 26 for theprocess material) is determined by calculating the mid-point between thefirst point 206 and the second point 208. It will be understood that thenumber of points in the pattern need not be limited to four. Additionalpoints could be used without departing from the scope of the presentinvention.

It is well known that variations in operating conditions such as;environmental variations, (temperature, humidity, pressure,) powersupply variations (voltage, current, power) electromagnetic influences(rf/uwave radiated power creating biases on IC outputs) and otherconditions such as mechanical vibration can induce undesired drifts ofelectronics parameters and output signals.

In order to compensate for drifts in time and voltage in reflectedsignals due to the above-described variations in operating conditions, afurther embodiment of the present invention includes a correctiveelement or factor that is calculated every time the software executes asignal processing loop. The correction element or factor is then addedto each signal sample prior to use of the baseline subtraction methoddescribed previously.

Referring to FIG. 10, an initial boundary or probe map time alignedsignal 220 that has been digitized and store in a microprocessor isshown. This signal 220 corresponds to signal 62 shown in FIG. 3. Thesignal 220 is time aligned relative a starting voltage V_(min) which islocated on the starting center line 222 of the negative going component224 of the signal 220.

FIG. 11 illustrates a situation where the real time TDR signal 226 hasdrifted in both time and voltage relative to the initial boundary signal220. When the baseline procedure is used in this situation, the resultswill not be valid. This invalid result can be overcome and corrected tocompensate for these signal drifts using the correction element orfactor according the present invention. The real time TDR signal 226 hasa new center line 228 which has shifted in time Δt_(i) and has shiftedin voltage Δv_(compi).

The compensation can be accomplished by obtaining the time and voltagevariations Δt_(i) and Δv_(compi) and adjusting the digitized real timeTDR signal 226 by the drift Δt_(i) and Δv_(compi). The correction factorV_(corr) is calculated by subtracting a specific, point 230 on thenegative-going component 224 of the initial boundary of the probe mapsignal 220 from its corresponding point 232 on the negative-goingcomponent 234 of the real-time TDR signal 226, then inverting the resultusing the 2's complement technique. This yields a number V_(corr) thatis always added to the real time TDR signal 226, regardless of offsetpolarity of the signals 220 and 226. The correction factor V_(corre) isrepresented algebraically by the formula: ##EQU1##

The compensated sample point V_(comp) (i.e. the center of the validsignal) is determined by the formula: ##EQU2##

The baseline procedure can be performed upon completion of thiscompensation in time and voltage. The resulting baseline signal is shownin FIG. 12. This compensated result provides a valid reflection pulsethat is easily analyzed providing the desired valid and accurateΔt_(valid).

In order to implement the pattern recognition technique and thecorrection factor shown illustrated in FIGS. 9-12, the softwareprogrammed in the microprocessor 46 is modified as shown in FIGS. 13 and14. FIGS. 13 and 14 illustrate the additional steps performed by themicroprocessor 46 as a result of the software modifications. Theadditional steps are shown inserted in the appropriate locations withinthe steps illustrated in FIG. 8. Thus reference numerals in FIGS. 13 and14 corresponding to reference numerals in FIG. 8 are intended to denotethe same steps. Further, although not shown in FIGS. 13 and 14, it willbe understood that the remainder of the steps shown in FIG. 8 occurringbefore and after steps 110 and 130 respectively would be performed inconnection with the steps shown in FIGS. 13 and 14. Steps 136-140, steps120-126 and steps 114-118 would not be performed when using the patternrecognition technique. However, the correction factor could be usedwithout the pattern recognition technique in which case all of the stepsin FIG. 8 may be performed.

Referring to FIGS. 13 and 14, the step for calculating and adding thecorrection factor is shown in block 250 and is performed between blocks112 and 128 in the process illustrated in FIG. 8. A more detailedbreakdown of the steps performed in block 250 is shown in FIG. 14.

Referring to FIG. 14, after the microprocessor 46 provides a timealignment of the TDR signal in block 112, the microprocessor 46 thensubtracts the specific point 230 on the initial boundary signal 220 fromthe corresponding point 232 or the real-time signal 226 in block 252 inaccordance with the formula set forth above. In block 254, themicroprocessor 46 then uses the 2's complement technique on the negativedifference value between points 232 and 230.

After the 2's complement technique is applied then the correction factorV_(corr) determined in block 252 is added to the uncompensated samplepoint of the real time TDR signal to produce a value of the compensatedsample point V_(comp). Thereafter, the microprocessor 46 calculates abaseline (BL) signal by subtracting the initial boundary signal from thetime aligned and corrected TDR signal to produce the baseline signalillustrated in analog form in FIG. 12. It will be understood that afterblock 123 the microprocessor 46 may proceed to block 136, block 120,block 114 or use the pattern recognition technique as shown in FIG. 13at 260.

Using the pattern recognition technique the microprocessor 46 first usesthe 2's complement technique on the negative-going component 204 of thebaseline signal 200 (See FIG. 9) in block 262. Thereafter themicroprocessor 46 searches for the predetermine four (4) point pattern(determined based upon the width 218 of the signal) in block 264 asshown in FIG. 9. If the predetermined pattern is not found then themicroprocessor 46 continues to search baseline signal samples until avalid pattern is found. This step is performed in block 266. Once avalid patter is found, then the microprocessor 46 determines a locationof the positive maximum value of the valid baseline signal in block 130shown in FIG. 8.

Although the invention has been described in detail with reference to acertain preferred embodiment, variations and modifications exist withinthe scope and spirit of the present invention as described and definedin the following claims.

What is claimed is:
 1. A method for processing a time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a valid output result corresponding to a process variable in a vessel, the method comprising the steps of:establishing an initial boundary signal before the process variable is located in the vessel; storing the initial boundary signal; detecting the TDR signal; determining a baseline signal by subtracting the initial boundary signal from the TDR signal; establishing a signal pattern having a time range based on the width of reflection pulses in the baseline signal; comparing the baseline signal to the signal pattern until a reflection pulse in the baseline signal matches the signal pattern; determining a maximum value of the reflection pulse that matches the signal pattern; and calculating an output result based on the maximum value.
 2. The method of claim 1 further comprising the steps of determining point on the initial boundary signal and a corresponding point on the TDR signal and calculating a correction factor by subtracting the point on the initial boundary signal from the corresponding point on the TDR signal.
 3. The method of claim 2 further comprising the step of adding the correction factor to the TDR signal to establish a valid TDR signal prior to determining the baseline signal.
 4. The method of claim 1 further comprising the step of establishing a threshold voltage prior to comparing the baseline signal to the signal pattern.
 5. The method of claim 4 further comprising the step of inverting negative-going components of the reflection pulses to positive-going components.
 6. The method claim 5 wherein the step of establishing a signal pattern includes the step of determining at least four points within the time range in proximity to the threshold voltage.
 7. The method of claim 6 wherein the step of comparing the baseline signal to the signal pattern includes the step of searching for a reflection pulse where the four points in proximity to the threshold voltage occur within the time range.
 8. The method of claim 3 further comprising the step of inverting the correction factor to a positive value prior to adding the correction factor to the TDR signal to establish the valid TDR signal.
 9. A method for processing or time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a valid output result corresponding to a process variable in a vessel, the method comprising the steps of:establishing an initial boundary signal before the process variable is located in the vessel; storing the initial boundary signal; detecting the TDR signal; determining a point on the initial boundary signal and a corresponding point on the TDR signal; calculating a correction factor by subtracting the point on the initial boundary signal from the corresponding point on the TDR signal; adding the correction factor to the TDR signal to establish a valid signal; determining a baseline signal by subtracting the initial boundary signal from the valid TDR signal; determining a maximum value of the baseline signal; and calculating an output result based on the maximum value.
 10. The method of claim 9 further comprising the step of inverting the correction factor to a positive value prior to adding the correction factor to the TDR signal to establish the valid TDR signal.
 11. The method of claim 9 further comprising the steps of establishing a signal pattern having a time range based on the width of the reflection pulses in the baseline signal and comparing the baseline signal to the signal pattern until a reflection pulse in the baseline signal matches the signal pattern.
 12. A method for processing a time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a valid output result corresponding to a process variable in a vessel, the method comprising the steps of:establishing an initial boundary signal before the process variable is located in the vessel; storing the initial boundary signal; detecting the TDR signal; determining a point on the initial boundary signal and a corresponding point on the TDR signal; calculating a correction factor by subtracting the point on the initial boundary signal from the corresponding point on the TDR signal; adding the correction factor to the TDR signal to establish a valid TDR signal; determining a baseline signal by subtracting the initial boundary signal from the valid TDR signal; establishing a signal pattern having a time range based on the width of the reflection pulses in the baseline signal; comparing the baseline signal to the signal pattern until a reflection pulse in the baseline signal matches the signal pattern; determining a maximum value of the reflection pulse thus matches the signal pattern; and calculating an output result based on the maximum value.
 13. An apparatus for processing a time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a value output result corresponding to a process variable in a vessel, the apparatus comprising:means for establishing an initial boundary signal before the process variable is located in the vessel; means for storing the initial boundary signal; means for detecting the TDR signal; means for determining a baseline signal by subtracting the initial boundary signal from the TDR signal; means for establishing a signal pattern having a time range based on the width of reflection pulses in the baseline signal; means for comparing the baseline signal to the signal pattern until a reflection pulse in the baseline signal matches the signal pattern; means for determining a minimum value of the reflection pulse that matches the signal pattern; and means for calculating an output result based on the maximum value.
 14. An apparatus for processing a time domain reflectometry (TDR) signal having a plurality of reflection pulses to generate a valid output result corresponding to a process variable in a vessel, the apparatus comprising:means for establishing an initial boundary signal before the process variable is located in the vessel; means for storing the initial boundary signal; means for detecting the TDR signal; means for determining a point on the initial boundary signal and a corresponding point on the TDR signal; means for calculating a correction factor by subtracting the point on the initial boundary signal from the corresponding point on the TDR signal; means for adding the correction factor to the TDR signal to establish a valid signal; means for determining a baseline signal by subtracting the initial boundary signal from the valid TDR signal; means for determining a maximum value of the baseline signal; and means for calculating an output result based on the maximum value. 